Mobile, gantry-type cranes are well known for lifting and moving large bulky objects such as shipping containers, heavy machinery, boats, or the like. Typically such mobile cranes must have sufficient maneuverability both to position themselves over the object to be lifted, as well as to place the object at its desired location. For example, such cranes must be able both to move longitudinally up and down an isle, as well as to move transversely between isles.
Voelz, U.S. Pat. No. 4,457,389, discloses a mobile gantry type crane providing both longitudinal, as well as transverse, movement. However, there is no directional control in the transverse direction.
Marine Travelift, assignee of this application, developed a directional control system for a gantry crane which provided limited directional control in the transverse direction. The crane has four spaced apart wheels, specifically a right-front wheel RF, a left-front wheel LF, a right-rear wheel RR and a left-rear wheel LR, as defined with respect to movement in the longitudinal direction. When moving in the longitudinal direction, steering wa accomplished by conventionally rotating the right-front RF and left-front LF wheels. The right-rear RR and left-rear LR wheels remained fixed. When switching from movement in the longitudinal direction to movement in the transverse direction, all four wheels (RF, LF, RR, LR) were rotated 90.degree.. Once all four wheels were rotated 90.degree., the right-rear RR and the left-rear LR wheels, which were now, in effect, the two right-side wheels with respect to movement in the transverse direction, could be rotated .+-. 5.degree. by means of a separate tiller control. This system provided maneuverability in the transverse direction. However, because steering was being accomplished by a side pair of wheels rather that a front pair of wheels, there was a tendency for the wheels not to properly track, resulting in increased tire wear. Further, the separate tiller control for transverse steering made control difficult.
The present invention is provided to solve these and other problems.